1. Field of the Invention
This invention relates to an anti-skid control system for achieving a satisfactory rotational behavior of the wheels of a motor vehicle during a braking operation. More particularly, this invention pertains to an anti-skid control system which is designed to increase and reduce brake fluid pressure in a desirable manner to prevent the wheels from becoming locked.
Furthermore, the present invention is also directed to an anti-skid control system capable of preventing wheel locking, which tends to occur when the motor vehicle is running on a road surface with a very low coefficient of friction.
2. Description of the Related Art
There have been heretofore proposed a variety of anti-skid control systems wherein, during a braking operation, the brake fluid pressure is reduced to prevent the occurrence of an undesirable wheel lock, and the brake fluid pressure thus reduced is then increased to avoid an undesirable extension of the brake stopping distance. Among such conventional anti-skid control systems is one wherein different pressure buildup rates are stored; a desired one of the stored pressure buildup rates is selected; and brake fluid pressure is increased with the selected buildup rate, for example. To effect the pressure buildup rate selection, a system has been proposed, wherein a solenoid-operated valve adapted to be opened and closed with a relatively high frequency is provided in the brake fluid pressure buildup system; a pulse train generator such as a multivibrator is provided in the drive system for the solenoid-operated valve; and the timing with which the solenoid-operated valve is driven by a pulse train signal derived from the multivibrator is controlled on the basis of a signal representing wheel acceleration (British patent specification No. 1305430).
Nevertheless, a more satisfactory anti-skid control is difficult to achieve with the aforementioned conventional system wherein the anti-skid control operation is performed by the opening and closing of a solenoid-operated valve which is driven by a pulse train signal either with a single average pressure buildup rate or a rate selected from several different pressure buildup rates. This will be explained in the following discussion. The decrease and recovery of the wheel speed requiring anti-skid control is influenced by various factors, such as the conditions of the road surface on which the motor vehicle is running, the coefficient of friction of the road surface, the vehicle running speed, the brake operating condition, the tire surface condition, the internal pressure of the tire tube, and so forth, and the overall effect of these factors is constantly and successively changing. The above-described conventional systems, however, fail to respond quickly enough to cope with such a situation, and this difficulty becomes most remarkable with the brake fluid pressure buildup mode for an anti-skid control which requires a delicate balance between two contradictory requirements, i.e., a requirement to restrain a re-locking of a wheel as effectively as possible and a requirement to prevent an undesirable extension of the brake stopping distance.